userx2
Full Member level 3
Hello friends
I am trying to measure high speed USB signals with an oscilloscope.
The signals are differential and run at 480Mb
The impedance of the tracks is 90 Ohm (Zdiff)
Problem:
I have 4 proto boards (10 layer) rev 1 and all working on USB2 high speed 100%
I now have a heap of new new proto boards (same design different board maker) rev 2and none work on USB2 high speed.
They claim the impedance has been matched and sent me the report and blah.
So I need to try and find the problem by measuring something.
I have a suitable scope with 50 Ohm inputs but no probes (they cost a fortune!!)
I tried to made 2 'probes' with RG58 coax and a series resistor of 1k, soldered directly onto the USB conenctor on the pcb. I soldered the shields together and grounded them at the pcb connector.
The probes are length matched as well.
This does measure but I found that the USB communication stops working and gets full of errors (on a working board).
It gets better when I unplug my probes' from the scope inputs but of course I then can't measure.
I upped the series resistors to 1k2 and this helped a little but still not satisfactory.
At 2k2, the scope no longer measures any usable waveforms.
My question, what am I doing wrong? I would have thought that even a 1k series resistor on the probe input should not load the signals enough to start introducing errors.
I have a (borrowed $$$$) 12GHz differential probe and that works great but I cannot see any eye patterns and other abnormalities with that.
Is there anything else I can do (cheaply) to measure these signals without the problems as above?
Apart from getting the boards re-made somewhere else, I am actually out of ideas on how to find this problem.
Interstingly, my USB analyzer sees errors on packets from my pch host side as well as from the device (memory stick) that is conencted to my host.
Best regards
X
I am trying to measure high speed USB signals with an oscilloscope.
The signals are differential and run at 480Mb
The impedance of the tracks is 90 Ohm (Zdiff)
Problem:
I have 4 proto boards (10 layer) rev 1 and all working on USB2 high speed 100%
I now have a heap of new new proto boards (same design different board maker) rev 2and none work on USB2 high speed.
They claim the impedance has been matched and sent me the report and blah.
So I need to try and find the problem by measuring something.
I have a suitable scope with 50 Ohm inputs but no probes (they cost a fortune!!)
I tried to made 2 'probes' with RG58 coax and a series resistor of 1k, soldered directly onto the USB conenctor on the pcb. I soldered the shields together and grounded them at the pcb connector.
The probes are length matched as well.
This does measure but I found that the USB communication stops working and gets full of errors (on a working board).
It gets better when I unplug my probes' from the scope inputs but of course I then can't measure.
I upped the series resistors to 1k2 and this helped a little but still not satisfactory.
At 2k2, the scope no longer measures any usable waveforms.
My question, what am I doing wrong? I would have thought that even a 1k series resistor on the probe input should not load the signals enough to start introducing errors.
I have a (borrowed $$$$) 12GHz differential probe and that works great but I cannot see any eye patterns and other abnormalities with that.
Is there anything else I can do (cheaply) to measure these signals without the problems as above?
Apart from getting the boards re-made somewhere else, I am actually out of ideas on how to find this problem.
Interstingly, my USB analyzer sees errors on packets from my pch host side as well as from the device (memory stick) that is conencted to my host.
Best regards
X